A common feature of a number of low energy using plants and processes for ammonia production is that at some stage in the process the synthesis gas contains excess nitrogen and is subjected to a nitrogen separation stage. The excess nitrogen is removed in various ways e.g. from the synthesis gas before the ammonia synthesis loop or from the purge gas with an arrangement allowing hydrogen from the purge gas to be recycled to the synthesis stage. Examples of processes employing a nitrogen separation stage are disclosed in U.S. Pat. Nos. 3,442,613, 4,298,588 and 4,409,196.
In the air partial oxidation process (APO process) disclosed in U.S. Pat. No. 4,409,196 a feed gas stream for ammonia is produced by the steps of
(a) partially oxidising oil, coal, natural gas or any combination thereof in the presence of air generally at a pressure of 15 to 150 bar and at a temperature of 300.degree. C. to 2000.degree. C. to produce a raw gas stream containing hydrogen and nitrogen with a stoichiometric excess of nitrogen of at least 200 mole percent based upon that needed for ammonia synthesis, together with carbon oxides, methane and hydrogen sulphide if sulphur was present in the oil, coal or gas,
(b) treating the raw gas stream from step (a) to remove substantially all component gases other than hydrogen and nitrogen,
(c) drying the raw gas stream from step (b) if water is present,
(d) subjecting the raw gas stream from step (c) to a nitrogen separation stage e.g. in a cryogenic separator, to separate (1) a hydrogen-nitrogen feed gas stream having a predetermined hydrogen:nitrogen ratio suitable for ammonia synthesis and (2) a nitrogen-rich gas stream and
(e) injecting the hydrogen nitrogen gas stream (1) into a reactor for ammonia synthesis.
The nitrogen-rich gas stream may be heated and then expanded in a turbine to generate power.
In one embodiment of the APO process the raw gas stream from the partial oxidation is passed over a shift catalyst and reacted with steam at elevated temperature to convert carbon monoxide present in the raw gas stream to carbon dioxide and hydrogen. The carbon dioxide content of the gas stream is then removed together with any hydrogen sulphide e.g. by scrubbing with hot potassium carbonate and thereafter the gas stream is subjected to a methanation stage to remove any residual carbon oxides. In the methanation stage the carbon oxides are reacted with hydrogen on a catalytic surface to produce methane and water. Purification of synthesis gas in this way to remove the residual carbon oxides is wasteful because the reaction consumes hydrogen which has been expensively produced in the upstream steps and is required for ammonia synthesis. After methanation the synthesis gas is dried and fed into the reactor.
In an alternative embodiment, in order to limit the loss of hydrogen due to methanation, only a fraction of the synthesis gas is methanated, e.g. 20 to 40% of the total gas stream. The remaining and larger gas stream fraction is then dried and subjected to removal of the residual carbon dioxide by adsorption, while the smaller fraction is dried. After partial liquifaction this smaller fraction provides carbon monoxide free liquid wash to the remaining portion of the feed gas in the wash column. The two synthesis gas fractions are fed separately to the cryogenic seperation plant.
U.S. Pat. No. 4,298,588 discloses an ammonia production process (AMV process) which comprises:
(a) primary catalytically reforming at superatmospheric pressure of a hydrocarbon feedstock with steam in order to produce a gas containing carbon oxides, hydrogen and methane; PA0 (b) secondary catalytically reforming the gas from step (a) by introducing air and bringing the mixture towards chemical equilibrium, whereby to produce a gas containing nitrogen, carbon oxides, hydrogen and a decreased quantity of methane; PA0 (c) converting carbon monoxide catalytically with steam to carbon dioxide and hydrogen; PA0 (d) removing carbon oxides to give a nitrogen-hydrogen ammonia synthesis gas and compressing said gas to ammonia synthesis pressure; PA0 (e) reacting the synthesis gas to produce ammonia and recovering ammonia from the reacted gas; and PA0 (f) discarding non-reactive gases present in the synthesis gas;
in which:
step (a) is conducted at a pressure of 40-80 bar absolute and in conditions of steam-to-carbon ratio and temperature to produce a gas containing at least 10% v/v methane and using in step (b) a quantity of air in excess of what would introduce 1 molecule of nitrogen per 3 molecules of hydrogen; and PA1 the reacted synthesis gas is treated to remove ammonia and to separate a gas stream enriched in hydrogen and the hydrogen-enriched stream is returned to the ammonia synthesis. PA1 pressure energy of the purge gas can be more effectively utilised by isentropic expansion in a turbine PA1 elimination of the requirement to methanate wholly or partially the synthesis gas from the APO derived process, because the liquified portion of the purge gas can be used to provide the washing function required to remove carbon monoxide from the synthesis gas liquified portion of the purge gas can be used to provide the washing function required to remove carbon monoxide from the synthesis gas PA1 the waste gases from the plant are able to emerge at a higher pressure, which is advantageous elsewhere in the synthesis gas preparation process PA1 hydrogen recovery from the purge gas is accomplished in order to satisfy the material balance of the process a a whole. PA1 APO synthesis gas 1.031 PA1 AMV synthesis gas 2.49
The hydrogen separation treatment in the AMV process can be by any suitable means, for example by cryogenic fractionation, molecular sieve adsorption of gases other than hydrogen or palladium membrane diffusion. The hydrogen stream returned to the synthesis can be substantially (over 90% v/v) pure but in any event gas recycled to the reactor should contain at least 3 molecules of hydrogen per nitrogen molecule. The non-reactive gases discarded from the hydrogen separation treatment in a side stream should of course be substantially free of hydrogen, since any discarded hydrogen represents wasted energy. If the side stream contains methane, the separation treatment can be designed and operated to separate a methane-rich element and that element can be used as process feed or furnace fuel for step (a) or feed to step (b). A typical side stream flow rate is in the range 15 to 30% of total gas flow.
A cryogenic purification of the hydrogen stream in the AMV process comprises the stages:
(a) subjecting it in a first indirect cooling stage to heat exchange with one or more cool streams to be described;
(b) cooling the product of stage (a) by expansion in an engine:
(c) subjecting the engine effluent in a second indirect cooling stage to heat exchange with one or more cold streams to be described, whereby to decrease its temperature to below the dewpoint of nitrogen;
(d) separating a liquid phase containing nitrogen, methane and possibly noble elements;
(e) passing the hydrogen-enriched gaseous phase resulting from step (d) into heat exchange in stage (c) as one of the cold streams,
(f) passing the hydrogen-depleted liquid phase from stage (d) into heat exchange in stage (c) as one of the said cold streams; streams thus warmed in stages (e)
(g) passing the streams thus warmed in stages (e) and (f) into heat exchange in stage (a) as the said cool streams;
(h) passing the hydrogen enriched gaseous phase back to the synthesis loop.
Usually the hydrogen-depleted phase will evaporate in stage (f) and possibly in part in stage (g). It is then discarded possibly by discharging it to atmosphere or by possibly using it as an auxiliary coolant or as a working fluid in a heat engine or as a fuel, depending on its composition and on local requirements. If its methane content is high enough it may be used for synthesis gas generation.
The present invention provides an alternative ammonia production process in which excess nitrogen is separated from both the synthesis gas stream before ammonia synthesis and from the purge gas which results from ammonia synthesis.